An interest in applying behavior genetic techniques to family research and an understanding of the gaps in the sibling and marital literature led to this study of genetic and environmental factors contributing to associations between personality, sibling and marital relationships and adjustment in adults. Three samples from the Swedish Twin Registry will be used to investigate the role that personality plays in associations between adult interpersonal relationships and adjustment: Young Twins, Twin Moms, and the Swedish Adoption/Twin Study of Aging. Multivariate analyses will be used to address the specific aims of this study: 1) examine associations between adult interpersonal relationships and adjustment across the lifespan by a) identifying genetic and environmental influences underlying associations of adult sibling and marital relationships with adjustment, and b) examining genetic and environmental on the influence of interactions between adult sibling and marital relationships on adjustment: 2) employ genetically informed designs to specify how personality characteristics influence interpersonal relationships and ensuing adjustment. A review of the family research and behavior genetic literature indicated the following set of expectations. 1) Prior findings of associations between sibling relationships and adjustment will be replicated. Shared environmental influences may be important. but it is not entirely clear how genetic and environmental factors will contribute to these associations. 2) Previous findings of associations between marital quality and mental health will be replicated. It is expected that these associations will be explained by genetic and nonshared environmental factors. 3) The proposed analyses will contribute the first findings on associations between adult sibling relationships and marital relationships, and the genetic and environmental factors contributing to the association. 4) The proposed study will report the first findings on how personality affects sibling relationships in later life. Findings are expected to replicate previous findings of links between personality and marital relationships. Contributions will also be made to the debate on intrapersonal models vs. interpersonal models of influence.